The use of explosives in public works and mining is so widespread that performing said activities without using them would be inconceivable today. Given the nature of these products and the amounts used, safety aspects both in their handling and transport to the site of use are very important and form a very important area of activity in the research and development of these technologies.
The market has evolved from using generally detonator-sensitive products packed in cartridges to using much less sensitive bulk products that must be initiated with a booster. “On-site” manufacture or sensitization is favored to facilitate transport to the site of use.
The earliest patents relating to “on-site” explosive manufacture, i.e., the manufacture of the explosive in the same truck used for unloading the explosive into the blast holes, were filed by IRECO (U.S. Pat. Nos. 3,303,738 and 3,338,033). These patents describe the manufacture of a water-gel-type explosive in a truck by means of metering and mixing a liquid solution containing oxidizing salts with a solid material containing oxidizing salts and thickeners. U.S. Pat. No. 3,610,088 (IRECO) describes the same method as the preceding patents for the “on-site” manufacture of a water-gel, incorporating the simultaneous addition of air either by means of mechanical trapping or by means of generating a gas through a chemical reaction. Patent EP 0 203 230 (IRECO) describes a blender having mobile and fixed blades allowing the “on-site” manufacture of a water-in-oil emulsion-type blasting agent.
The greatest drawback of these earliest “on-site” manufacturing technologies lies in the fact that they use high-temperature oxidizing salt solutions that must be transported with a heat supply in thermally insulated tanks. The complexity of the truck and of the manufacturing operation requires highly qualified staff to assure its success.
The emergence of emulsions changed the trend towards the transport of matrix emulsions classified as non-explosive emulsions and their “on-site” sensitization either by means of mixing with hollow microspheres or by means of generating gas through a chemical reaction. Based on the same philosophy, MAXAM (formerly known as Unión Española de Explosivos) developed a series of technologies based on the transport of a non-explosive matrix suspension and its “on-site” sensitization by means of incorporating air to the matrix before unloading it into the blast hole.
European patent EP1002777 B1 (MAXAM, formerly known as Unión Española de Explosivos) describes a method and an installation for the “on-site” sensitization of water-based explosives before loading the blast holes from a non-explosive matrix suspension. The sensitization is carried out by means of mixing metered amounts of the matrix product with a gas or air and a gas bubble stabilizer. Likewise, European patent EP1207145 B1 (MAXAM, formerly known as Unión Española de Explosivos) discloses a method for the “on-site” manufacture of water-based explosives before loading the blast holes from an oxidizing matrix suspension with an oxygen balance greater than +14%, a fuel material, a gas or air and a gas bubble stabilizer. U.S. Pat. No. 6,949,153 B2 (MAXAM, formerly known as Unión Española de Explosivos) describes a method for the “on-site” manufacture of pumpable explosive mixtures by means of mixing a granular oxidizer with a non-explosive matrix suspension stabilized with a thickener, air and a gas bubble stabilizer which allows regulating the density of the end product according to the process conditions. This method allows controlling the density of the explosive product before unloading into the blast holes by means of the controlled incorporation of atmospheric air by mechanical means.
Another alternative is the transport of the matrix product and its sensitization at the site of use by means of mixing the matrix with low-density granulated nitrates or with the mixture of ammonium nitrate with a liquid hydrocarbon (ANFO). U.S. Pat. No. 4,555,278 and EP 0 194 775 describe explosives of this type formed from emulsions and water-gels, respectively. The sensitization in such explosives, known as “heavy ANFOs”, is due to the actual porosity of the porous ammonium nitrate granules and to the entrapped air between the gaps thereof. Such mixtures are not pumpable, the blast holes are loaded by means of augers and their water resistance is very limited. The nitrate particle content is generally greater than 50% given that for lower contents the resulting mixture is very dense since the liquid matrix occupies the spaces between the granules, the mixture having too low initiation sensitivity.
The use of explosives in mining or public works may lead to the event where, due to the characteristics of the rock and/or of the geological structure of the terrain, the optimal explosive that must be used has to have a low-density (0.4-0.8 g/cm3) and low detonation velocity (2-4 km/s). ANFO is the most frequently used explosive even though it is included in the higher end of the density range (0.8 g/cm3). When the density of the ANFO is to be reduced, it is mixed with a low-density granular material which can be inorganic and therefore inert, or organic, and in this case it also has a fuel function. The use of standard or low-density ANFOs is limited only to the case of dry blast holes because these explosives are not water-resistant.
When blast holes contain water, heavy ANFOs (mixtures of matrix and ANFO with a high ANFO content) or doped emulsions (mixtures of matrix and ANFO with a low ANFO or granular nitrate content) are normally used. In the first case, the resulting explosive has a density greater than that of the ANFO because the emulsion is located in the space between the ANFO granules. This is also why the water resistance is very limited and the prolonged stay of the explosive into the blast hole can cause the gases originating from the subsequent detonation thereof to have a high nitrogen oxide (red smoke) content.
In the case of doped explosive emulsions, the resistance of the explosive to water is assured due to the excessive emulsion. However, this solution has a serious drawback. If the matrix emulsion is sensitized by means of chemically generating gas bubbles and the final density of the explosive is therefore controlled by the total volume of these bubbles, the average density of the explosive into the blast hole is generally not very low, and the density will be higher the greater the height of the blast hole. Due to the hydrostatic pressure along the explosive column in the blast hole, gas from bubbles located at the bottom of the blast hole is highly compressed and the density of the explosive is relatively high in the bottom part of the blast hole. To compensate this effect, the volume of gas generated is increased by means of chemical gassing, resulting in an explosive with a very low-density at the top part of the blast hole. However, this solution is very limited because an excessively low density at the top part of the blast hole causes a very significant reduction in the consistency of the final explosive, leading to the collapse of the explosive column or facilitating the introduction of the stemming material in the explosive column. This phenomenon prevents being able to achieve relatively low average densities in the blast hole by means of this solution. The solution used for reducing the density in these cases consists of adding very low-density solid particles to the emulsion. This option in turn has other drawbacks in addition to a significant raw material cost increase. If these particles are added to the matrix in the factory, the matrix is no longer non-explosive, and a bulk explosive must therefore be transported. If in contrast these hollow particles are added “on-site”in the truck, the truck to be used is more complex and has smaller capacity due to the considerable volume of the compartment containing the solid density-reducing agent and to the actual metering thereof.